scxi 1600 4-20 ma

[Gryphon]

Hi All,

I have been having some trouble with a pressure transducer and the SCXI-1600 with SCXI-1303 connection module.

The pressure transducer is a 2 wire industrial transmitter.  24V is connected to the + of the sensor and the - of the sensor is connected to one end of a 150 ohm resistor.  The other end of the resistor is connected to ground.  The sensor varies the current in proportion to the pressure and the range of the current is 4-20mA.

I have verified that the sensor works correctly with a multimeter in series from the power supply to the sensor. I can see that the current flowing is just over 4mA as expected.  On the other end, I can measure the voltage across the resistor is ~600mV when the resistor is connected directly to ground.

I am connecting this sensor to input 0 of an SCXI-1303 module.  The wiring is as follows

24V ---- Sensor +ve

Sensor -ve ----------------CH0+

CH0+ --------- 150 ohm resistor

150 ohm resistor --------- CH0-

CH0- -------- GND

Basically the 150 ohm resistor is connected across CH0 + and -.  The negative wire from the sensor goes the CH0+ and the power supply Gnd is connected to CH0-.  The current from the sensor should create a voltage across the resistor that the SCXI reads and I should get the pressure.

The problem is that when I connect a ground to CH0-, there is a current leakage such that the voltage across the 150 ohm resistor is only about 390mV instead of 600mV.  About 1.4mA appears to be lost.  If I disconnect the power supply ground from the CH0-, the 600mV is there across the resistor that is across the CH0 + and - terminals.

The measurements of 600mV and 390mV are what are measured with a multimeter.  The SCXI is not able to read the pressure correctly even though the input is configured to read current with the correctly specified shunt resistor and with a proper pressure scaling specified.

If I could measure what I expect across the terminals I would be drawn to a configuration error in the DAQ assistant but I am seeing incorrect behaviour across the terminals with a multimeter that has me worried.  

Any thoughts or suggestions would me much appreciated,

Cheers

Peter

[Courtney_L]

Hi Gryphon,

This connection set-up won't work. THe SCXI-1600 itself isn't going to acquire analog inputs from sensors -- rather, it is a control module for other SCXI modules. From the product manual:

"The SCXI-1600 receives analog signals from other SCXI modules and amplifies, digitizes, and sends data through a USB port. The SCXI-1600 can also control digital input, digital output, and analog output SCXI modules. The SCXI-1600 has no DIP switches, jumpers, or potentiometers. It is easily configured and calibrated using software. The SCXI-1600 connects to other SCXI modules to acquire up to 352 analog signals from thermocouples, RTDs, strain gauges, voltage sources, and current sources."

To use the SCXI-1600, you will need an additional module to read in an analog input, and then that input (from the other module) could be routed to the SCXI-1600. The SCXI-1303 is just a terminal block. You cannot just plug your pressure measurement sensor into the SCXI-1600 or SCXI chassis and read in the analog input that way. Do you have any other modules around that you could use for analog input? 

[Gryphon]

Hi Courtney,

thanks for your reply.  I feel pretty dumb because I mixed up the 1600 with the functionality of the terminal block.  

The actual SCXI module which the 1303 is plugged into is a SCXI -1102. 

I suppose the question I should be asking is if I can mix current inputs with thermocouple inputs with the 1303 on a 1102 module.  I don't think the 1102 minds but the 1303 with the default pull up and pull down resistors does not work.

I was able to get good current readings with a 1303 by removing all the network resistors. Would I be able to do this without compromising the thermocouple readings.  I understand I will lose the open thermocouple detection.

thanks,

Peter

[johnsold]

Peter,

Your thermocouple channels may not work without the bias resistors connected.  The 1102 needs the bias current they provide even if you do not care about the open thermocouple detection.  The resistor networks connect to blocks of 8 channels, two networks per block. Change the ch0 to ch7 block for the pressure transducer and leave the others for thermocouples (so long as you do not have more than 24 temperature channels).

I do not understand why your current measurement does not work.  You should be able to short the Ch- input to ground without any problems. The default resistors are 10 megohms but 10 ohm (and other value) resistor networks are also available.  I could see the 10 ohm resistors introducing errors, but it seems unlikely with the 10 megohm resistors.

Lynn

[Gryphon]

Hi Lyn,

thanks for your post and sorry for taking a while to get back, have been on the road.

This is hardware that my customer has canibalised from an old project and so I did not set up the resistors myself and I can't see any spares.  The resistors are marked 10x and I thought that this might mean 10 ohms instead of 10MOhms. 

I was planning on trying to replace the resistors with 10MOhms and I just pulled out the resistors because I did not have any 10M resistors on me.

Does anyone know what the markings are on the 10ohm and 10MOhm NI supplied resistors?

thanks,

Peter

[johnsold]

Peter,

We have some of each and some third party 1000 ohm resistor networks, but I will not have access to them before next week.

10x certainly sounds more like a 10 ohm marking.  10 ohms would explain some of the results you saw.

Edit: I looked at the SCXI1303 Install manual, page 8.  It shows the markings as 10x-1-100 for the 10 ohms and 10x-1-106 for the 10 megohm netwrok.

Lynn

[Gryphon]

Hi Lyn,

Will have to get back to site and check the resistors.  Thanks for the info on the markings,  I missed that on my read of the manual.  Will post when I replace the resistors with 10MOhm resistors.

cheers

Peter